1. Field of the Invention
The present invention is directed to an I/O relay interface module mounting a number of relays for coupling a central controller to a corresponding number of I/O devices through the respective relays.
2. Description of the Prior Art
In electrical systems controlling a number of external I/O devices by a central controller such as programmable controllers and single-board computers, there have been utilized relays for interface between the central controller and the individual I/O devices. In order to avoid complicated wiring between the central controller and the individual I/O devices through the respective relays and to avoid the attendant maintenance difficulty, it has been an industry practice to utilize an I/O relay interface module mounting the plural relays. The I/O relay interface module has a bus connector for communication with the central controller and a number of wiring terminals for connection with the individual external I/O devices. A time division multiplexing technique is utilized for the above communication between the central controller and the individual relays on the I/O relay interface module in order to control the I/O devices through the associated relays or to monitor the operations to the I/O devices through the associated relays. For such I/O relay interface module, there have been proposed prior art constructions as disclosed in Japanese Patent Publication (KOKAI) Nos. 63-88770 and 63-88771 (corresponding to U.S. Pat. No. 4,808,114 to Mohri et al). In the prior art patents, the I/O relay interface module is designed to only accept up to a fixed number of relays, for example, eight or sixteen relays on the module. With such constructions, the user have to utilize the interface module designed to have, for example, sixteen relays even when only a less number of, i.e., eight relays are required in an intended control system, which imposes undue cost to the system. On the other hand, it may be required after the initial completion of the system to add one or more relays with corresponding increase in the number of the I/O devices in the system. In such case, the user have to replace the existing I/O relay interface module with another module capable of mounting a more number of relays. This also results in undue increase in cost as well as incurs cumbersome operations of disconnecting and reconnecting the existing I/O device wiring.